Moving-coil sound recording head



1952 A. R. SUGDEN 2,606,254

MOVING-COIL. SOUND RECORDING HEAD Filed March 18, 1950 fig].

Patented Aug. -5, 1952 UNITED STATES PATENT OFFICE,

Arnold Robinson Sugden, Brighouse, England Application March 18, 1950, Serial No. 150,384 In Great Britain March 18, 1949 havebeen and are of what are known in the art as (moving iron type, in which an armature of magnetic material carrying a cutting stylus is disposed in the centre of a coil of wire located between the pole pieces of a magnet. An alternating current is passed through the coil causing the armature to be magnetisedythe polarity and magnitude being determined by the direction and magnitude of the current in the coil. The'reaction between the static field of the magnet and the alternating field set up by the coil is resolved into angular oscillations of the armature about a pivot point. As a result of these oscillations the stylus carried by the armature is caused to cut a groove on a record disc rotated beneath it at a constant angular velocity, the shape of the groove being representative, as regards frequency and magnitude, of the waveform being recorded.

The whole of the cutter assembly, coil and permanent magnets are mounted either on a head base plate or back plate, and then all pivoted at a suitable point and counterbalanced to reduce the downward pressure of the whole assembly to the pressure required for cutting the suitable depth in the wax or lacquer disc beneath.

As the disc surface is not perfectly fiat, the cutter assembly rises and falls in relation to the undulations of the disc.

It will be appreciated that when the heavy cutter head is set in motion, the irregular surface causes it to assume a momentum of its own, which again causes the cut to be of varying depths, which is known in the trade as patterning. To remedy this, various methods are employed, such as springs, counterbalancing, etc, in conjunction with a dashpot assembly to damp out this period of resonance. Obviously, the best method to reduce this defect would be to have just sufficient mass to create this downward pressure in order to obtain the correct depth of out without relying on counter-balancing or equivalent means.

Also, the best type of cutter head is what is known as the moving coil, but this has not been a feasible proposition due to the large size of magnet necessary to give the high flux required across the coil assembly. In the new type of cutter head and assembly which constitutes my invention, all the aforementioned points have been taken care of.

2 Claims. (01. 1'79100.41)

The invention is characterised broadly in that, instead of the magnet assembly being carried as hitherto by the cutter head proper, that is to say by the part carrying the cutting stylus, it is carried by the traverse head by means of which the cutter head is traversed radially across the record.

A constructional embodiment of the invention is shown in the accompanying drawings in which Fig. 1 is a side elevation of the improved cutter head arrangement;

Fig. 2 is a sectional plan view on the line'2-2 in Fig. 1;

Fig. 3 is a side elevation of the cutter-carrying platform; and v Fig. 4 is a detail elevation, looking in the direction of the arrow 4 in Fig. 1, with the cover.

plate of the magnet assembly removed.

Referring to the drawings, 42 represents the usual traverse head by means of which the cutting stylus b is caused to travel radially across the surface of a record 0.

Pivotally connected at d to a lug or, lugs such as a on the traverse head a. is a lug or lugs such as e on a magnet casing e containing a permanent magnet ,f having spaced pole pieces I, I, see Fig. 4. V

A platform 9, mounted pivotally between two pins h, h, carried by the magnet casing e, supports between depending brackets g, g, acoil i wound upon a former y of non-magnetic material, current being led to the coil by connectors 7", y", from terminals 7' 7' mounted on the platform 9. The former 1 is held in the brackets g, g by sleeves such as g of yielding material such as rubber, the arrangement being such that the cutter stylus b can modulate in relation to an A. C. signal passed through the coil 2'. As will be seen, the coil assembly can, by reason of its pivotal connection to the magnet casing, rise and fall, due to undulation of an irregular record surface, without leaving the magnetic field constituted between the faces of the pole pieces I, f, of the magnet I.

An adjustable screw m carried by the magnet casing e is adapted to bear with its lower end upon the traverse head a. By appropriate adjustment of this screw in the height of the cutting stylus can be adjusted.

To enable the depth of cut made in the record, the end of the platform 9, opposite to the axis about which it is pivotal, has a projection n to receive a weight or weights of appropriate amount.

The pivotal mounting of the magnet casing on the traverse head enables the cutter head to be swung upwardly to give access to the cutting stylus, should this require replacement.

The magnet assembly and the platform g carrying the cutting stylus b are traversed across the record disc 0 in the usual manner but owing to the employment of a moving coil system for operating the cutting stylus, and to the fact that the platform g carrying the coil 2' and stylus b can be made exeremely light owing to its capability of pivotal movement independently of the magnet system I am enabled to produce sound records in which the defects inherent in those produced by cutter heads of previously-known constructions are entirely avoided. Moreover, the improved cutter head is highly efiicient due to the small operating current required for the coil 1 when mounted in a high flux-density gap. 4 Having thus described my invention, what I claimes new and desire to secure by Letters Patent is: l

' 1. A cutter head for a sound recording apparatus comprising, in'combination, a traverse head adapted to'be radially moved with respect to the surface 'of a recorddisk; a magnet casing pivotally mounted adjacent one end thereof to said traversehead; a bolt threadedly engaging said niagnettasingat a point spaced from said p-ivotally mounted end thereof, said bolt bearing with one end thereof against said traverse head, whereby the turning of said .bolt causes the turning of said magnet casing with respect to said traverse head so as to adjust the position of said magnet casing with respect to said traverse head; a permanent magnet mounted in said magnet casing and having a pair of mutually spaced pole'pi'eces; an elongated platform pivotally connected adjacent one end thereof to said magnet casing andhaving the other end thereof located adjacent tosaid pole pieces; a coil support of non magnetic'material mounted on said other end of said platform, said coil support being located between said pole pieces; a coil of electrically conductive material mounted on said coil support and adapted to receive an electrical sigml; and a cutting stylus connected to said support, the pivotal mounting of said platform permitting'said cutting stylus to move up and down with irregular undulations in the surface of a record (disk While maintaining said coil in the field betwee said pole pieces so that said stylus may properly cut the disk.

2. A cutter head for a sound recording apparatus comprising, in combination, a traverse head adapted to be radially moved with respect to the surf-ace of a record disk; a magnet casing pivotally mounted adjacent one end thereof to said traverse head; a bolt threadedly engaging said magnet casing at a point spaced from said pivotally mounted end thereof, said bolt bearing with one end thereof against said traverse head, whereby the turning of said bolt causes the tuming of said magnet casing with respect to said traverse head so as to adjust the position of said magnet casing with respect to said traverse head; a permanent magnet mounted in said magnet casing and having a pair of mutually spaced pole pieces; an elongated platform pivotally connected adjacent one end thereof to said magnet casing and having the other end thereof located adjacent to said pole pieces; a coil support of non-magnetic material mounted on said other end of said platform, said coil support being located between said pole pieces; a coil of electrically conductive material mounted on said coil support and adapted to receive an electrical signal; a cutting stylus connected to said support, the pivotal mounting of said platform permitting said cutting stylus to move up and down with irregular undulations in the surface of a. record while maintaining said coil in the field between said pole pieces so that said stylus may properly cut the disk; and means for removmounting a. weight of desired magnitude on said otherend of said platform so as to adjust the pressure with which said stylus bears against a record to be cut.

ARNOLD ROBINSON SUGDEN.

REFERENCES CITED The following references are of record in the file of this patent: Y

UNITED STATES PATENTS Number Name Date 2,290,057 Lindenberg July 14, 1942 2,318,308 Harris May 4, 1943 2,367,846 De Sart Jan. 23, 1945 2,406,597 Faus Aug. 27, 1946 

